4L80E torque lockup problem

[45467]

I have the MPH set at 40 , rate 30 , delay 3.0 , lockup at wot.

[Steve Chryssos]

There's not a whole lot of lock up control in that TCOM version. MPH is obvious (although, that setting is kinda low); Rate relates to how quickly the TCC applies and releases--ramp rate. And delay prevents the lock up for re-engaging too quickly after release. 3 sec is fine if you think about how long it takes to apply the throttle, make a decision and release the throttle or vice versa. I would uncheck WOT until you resolve your issue and possibly leave it unchecked if you do not have a heavy duty lock up clutch engaging against a billet front cover. Anyway, none of this is adversely affecting the wacky apply setting.

I think I got it. Let's go over your harness connections:
-The underdash TCC connector should be looped into itself--not connected to a brake pedal. And double check that you haven't plugged in the 4L60 connector.
-On the old TCI design, you need to connect both VSS connectors at the trans. Propshaft (tailshaft) and turbine (TC). To my knowledge, Compushift is the only brand that ignores the turbine input.
-Confirm that trans type is set to 4L80E in your operational parameters.

Something like one of the above is confusing your TCU.

[45467]

Thank for the suggestion Steve, I checked everything out last night and everyting check out OK. Both vss are plugged in the right location , the 4L80 E connector is into the TCU unit , the 4l80E program is selected . I reset the lockup speed at 45 , rate at 65% delay 3 seconds & the WOT applu is off. Same results ! Could there be a mechanical problem within the transmission that could cause this ?

[Steve Chryssos]

Bummer. I was really hoping to win a kewpie doll for figuring it out. I'm a computer guy. I'm not qualified to diagnose the transmission, but as far as I can tell it's not the transmission. Your issue is that it's locking up at the wrong speed. The hardware (a pwm solenoid operating a piston) is fairly dumb. It does what it's told to do. And on the input side, VSS and TPS control part throttle shift tables as well as lock up, so if the input sensors were at fault, I think you would be experiencing larger issues--no shifting.

Try unplugging the 4l80E lock up connector where it loops back into the harness to close the circuit. Inspect both halves of that connection and reconnect. Or maybe the 4l80E and 4L60E lock up connectors are labelled backwards such that you presently have a mislabeled 4l60E lock up connector plugged in. I don't know if its harmful to reverse them.

[Jake's Performance]

OK,
Steve has you on the right track.
A 4L60E and 94-up 4L80E connector has the same connector but the pinout is slightly different.
A 4L80E doesn't use a 3-2 downshift solenoid like a 60 does and there are two other wires that have to be re-pinned to convert from a 4L60E to a 4L80E but I would have to look at my notes in the shop to remember which two they are and what they control.

I doubt it is the transmission as well.

You could tie into the TCU lockup wire and have it go through a switch to ground and see if you can command lockup at will to isolate the problem.

[45467]

The transmission used is a 1991 model, could it be wired differently than a 94 & up ? I checked the wiring harness from the TCU to trans at the TCU connector & the 4L80E /4L60E connectors are on the right color wire as shown on the supplied wiring diagram The wirinharness could still be wrong . I will trace each wire to the plugs at each end to make sure they emd up on the right pins.

[Jake's Performance]

Yes,
A 1991 is wired differently than a 1994-up.
Let me get you some info to help you out, you'll likely need to drop the pan.
Someone may have updated the wiring if it was rebuilt though.

[terrainman1]

I have a 1991 GMC 2500 Sierra.
The problem I have is that at 110km/hr the Transmission starts downshifting from overdrive then right back in and over and over. I had it recently into the transmission shop and after a full scan they said it was because the TPS is sending the driver demand downshift signal to the computer to shift the transmission. So I replaced both the TPS and the Computer and the problem persists.
This truck has been lifted 6 inches and has 35" tires on it now. Additionally the Speedometer was adjusted to match proper speed after the lift and tires were done.
Anyone have any suggestions what could be done to finally fix this problem. I am at my wits end and am tired of spending money with 0 results.